1. Field of the Invention
The present invention relates generally to froth flotation separation and has particular use in the field of deinking of paper in a process for recycling waste paper. More specifically, the invention concerns an apparatus and method for separating ink-laden foam from a slurry of the paper stock.
2. Description of the Related Art
Froth flotation is a subdivision of a general separation technique known as adsorptive bubble separation. In froth flotation, particles are selectively separated by the differing abilities of different particles to adhere to air bubbles rising through a bulk liquid, and to be held at the liquid surface in a foam or froth.
Flotation separation is frequently used in a variety of processes for various purposes. For example, selective flotation can be used to separate minerals, and total flotation is used for clarification by removing all solids. Recently, a developing application for flotation separation has been for ink removal in the field of paper recycling.
Various factors have contributed to development in the art of recycling waste papers for the purpose of repulping the paper to form paper. Recycling makes use of a readily available source of materials, the waste paper or discarded papers which would otherwise present disposal problems. Normally, this represents a very inexpensive and readily available source of material, thus providing an economic incentive for the papermaker to recycle waste paper. Increased ecological awareness has resulted in a demand by various populations to recycle paper, thereby eliminating the disposal problem and the resultant ecological impact. Therefore, there are incentives for the papermaker to encourage consumers to recycle paper and for the papermaker himself to use recycled fiber. Use of recycled fiber often can provide a less expensive paper product than using virgin fiber, and there is an economic incentive on the part of consumers to encourage papermakers to use recycled fiber as well as for the consumer himself to make available waste paper for recycling.
The process of waste paper recycling is essentially one of removing contaminants from usable fiber. In the deinking of wastepaper, which is a significant portion of the overall wastepaper recycling market, one of the critical steps is removing ink from a slurry of pulp fibers. Typically, the paper to be recycled is repulped in some fashion, and a slurry of the recycled fibers is made. Chemicals are added to separate the ink particles from the fiber, and to disperse the ink particles freely in the pulp slurry. Various different processes and chemicals can be used, which do not constitute a part of the invention and will not be described in further detail.
After the ink has been separated from the fiber it must be removed from the pulp slurry. Two processes for removing the ink particles from the slurry have been used by most recyclers, the selection of which often depends on various characteristics of the fiber being recycled and the ink to be removed. A first process for removing the ink particles is often referred to as dilution washing which essentially rinses the ink particles from the pulp.
A second type of ink separation is generally referred to as froth flotation. In froth flotation separation, pulp of low consistency is passed through a series of flotation cells. Air is mixed with the stock along with flotation chemicals such as fatty acids or surfactants. The ink particles have a greater affinity for the air bubbles than do the paper fibers, and the ink particles attach to the bubbles. The bubbles rise to the surface in the flotation cell, forming an ink-laden froth which can be continuously removed from the slurry surface. Removal of the froth is accomplished by gravity, by vacuum or with paddles or other skimming devices at the surface. The froth flotation process can also be used for removing various adhesive contaminants known as stickies from the pulp slurry as well.
Generally, less than thirty per cent of the total ink present in the slurry will be removed in a single froth flotation cell. Therefore, it has been a common practice in the past to provide four to six flotation cells in a series to remove an acceptable portion of the ink originally present in the repulped slurry. Each deinking cell requires various valves, pumps and associated apparatus including piping for providing slurry to the cell, the air injecting and mixing apparatus, chemical addition, froth removal and slurry removal. Therefore, the capital investment necessary for a system to remove the required portion of the ink from recycled fiber can be quite high.
Known froth flotation cells are all of an atmospheric type. That is, the slurry in the cell is at atmospheric pressure. The air injected into the slurry to float the ink particles is allowed to escape to the atmosphere. A typical example of the heretofore known froth flotation deinking cells can be found in U.S. Pat. No. 4,548,673. While a cover is often provided on the deinking cell, the cell remains to operate at atmospheric pressure. For example, U.S. Pat. No. 4,328,095 discloses a series of covered chambers; however, the chamber operates at atmospheric pressure.